For purposes of this application, the present invention is discussed in reference to polycrystalline materials, but the present invention is applicable to any heterogeneous material such as paracrystalline materials. A polycrystalline material is a material that is made of microstructure comprising many smaller crystallites, or grains, with varying orientation. The variation in direction of the grains, known as texture, can be random or directed depending on growth and processing conditions. The grains also vary in size, deformation (elongation), and void spaces between grains, or porosity.
A polycrystalline material includes almost all common metals and many ceramics. A polycrystalline material is a structure of a solid, for example, steel or brass, that when cooled form liquid crystals from differing points within the material.
One example of a polycrystalline material is steel. For exemplary purposes, the present invention is discussed in reference to steel in the form of railroad rail, but the present invention is applicable to any material in any form or size or shape for which material properties are desired to be determined and monitored over time such as to assess conditions of stress and defects.
Rail is used on railways, otherwise known as railroads, which guide trains without the need for steering. As shown in FIG. 1, rail tracks 20 typically consist of two parallel rails 22, 23. Rails are typically made from steel, which can carry heavier loads than other materials. Rails 22, 23 are laid upon cross ties 24 that are embedded in ballast 26. Cross ties 24, also known as sleepers, ensure the proper distance, or gauge, between the rails 22, 23. Cross ties 24 also distribute the load, or force, on the rails 22, 23 over the ballast 26. Plates 28 are positioned on top of cross ties 24 to receive rails 22, 23. The rails 22, 23 are then fastened to the cross ties 24 by a fastener 30, for example, with rail spikes, lag screws, bolts, or clips. The fastener 30 is driven through the plate 28 and into the cross tie 24.
Shown in FIGS. 2 and 3 is a representative rail 22. Rail 22 consists of rail sections 22′, 22″. Rail sections 22′, 22″ can be aligned and secured together by joint bars 32 (FIG. 2) or welding 34 (FIG. 3). Most modern railways use welding to align secure rail sections, known as continuous welded rail (“CWR”), to form one continuous rail that may be several miles long. In this form of track, the rails are welded together such as by thermite reaction or flash butt welding.
Longitudinal stress is a problem over large regions of rail track. Stress is a measure of force per unit area, typically expressed in pound-force per square inch (psi). The term “longitudinal” means “along the major (or long) axis” as opposed to “latitudinal” which means “along the width”, transverse, or across.
Longitudinal rail stress (“LRS”) is usually related to rail contractions and expansions due to changes in temperature. Longitudinal rail stress leads to failure, which is loss of load-carrying capacity. Examples of failure include, for example, buckling and fracture. Rail experiences tensile stress in cold temperatures, which can lead to fracture or separation of a rail into two or more pieces. In hot temperatures rail experiences compression stress, which can lead to buckling or warping. Tensile stress is a stress state causing expansion (increase in volume) whereas compression stress is a stress state causing compaction (decrease in volume). It should be noted that a zero stress state is when the material does not experience any stress. Failures, among other things, cause derailments and service disruption.
The ability to measure longitudinal rail stress is a primary challenge in the railway industry. The presence of large regions of rail track reduces the ability of rail to expand and contract easily due to daily and seasonal temperature changes. Thus, high longitudinal stresses can develop, which, in turn, leads to possible failure.
In the United States, from years 2001-2003, there were over 98 derailments associated with track buckling. Damage estimates for these derailments exceed $37 million. In addition, over 900 additional incidents associated with rail stress were reported. LRS is an on-going major difficulty for railroads.
There has been extensive research to develop a non-destructive method to measure LRS. Current techniques include strain gauges (e.g., available from Salient Systems) and rail uplift (e.g., the VERSE system by Vortok, Inc.). There are downfalls to these current techniques. Strain gauges only provide measurements related to stress in a local, or confined area. Additionally, strain gauges present difficulty in determining the zero stress state. Measurement by rail uplift is costly and requires a section of rail to be detached from the ties. Techniques, such as these single-point measurements, make it difficult to obtain measurements on large regions of rail track. Besides steel, a variety of other polycrystalline materials may need to be assessed to determine and monitor microstructural properties over time.
Traditional ultrasonic inspection methods include stress induced displacement, angle of incidence, differential pulse transit, pulse count, and signal relativity. With induced displacement, an ultrasound wave is introduced into the material using a transducer at a specified angle of incidence. The signal is received by an array of sensors at a predetermined spacing a distance from the transmitter after passing through and reflected from the bottom surface of the material. The spacing between the transmitter and receiving sensors is modified by a hydraulic servo-controller to maximize the signal at the center receiving sensor. Material height is measured independently in order to quantify the travel distance of the incident wave.
Angle of incidence introduces a longitudinal wave into the top surface of the material. The refraction path through the material and reflected from the bottom surface is measured by sensors. The angle of the transmitting transducer is adjusted to maintain a constant signal at the receiving sensors. This change in angle is used to determine the stress states of the material.
Differential pulse transit uses a pair of pulse trains coupled into the material. The time difference measured in the equivalent pulses in the pair of pulse trains is then related to the stress state of the material. The baseline travel time is based on measurements on stress free material. Differences in the travel time are indications of compressive or tensile stress.
The pulse count method introduces a pulse train into the material. The pulses are spaced such that the stress states will cause them to overlap or spread in time. The number of pulses is counted to extract the stress state provided the pulse separation is appropriately chosen.
Two successive sinusoidal waves are introduced into the material with signal relativity. As the waves propagate, their spacing in time changes based upon the stress state. This spacing is determined by quantifying the amplitude of the received signal relative to the incident waves.
Problems with these methods are that they all require introduction of the ultrasonic wave through the top surface of the material and reflection of the incident waves from the bottom surface of the material without considering the microstructural properties of the material.
An improved ultrasonic inspection system and methods are needed for any and all types of materials regardless of size and shape to assess microstructure properties of the material. Determining and monitoring material properties of microstructure over time may lead to specific types of processing of these materials in order to reduce or eliminate stress or defects in the material. For example, a specific sequence of a heat treatment process, such as annealing or sintering, may be utilized to alleviate significant alterations of microstructure during processing.
There is a demand, therefore, for an improved ultrasonic inspection method that is reliable, practical, and cost effective with which changes in microstructural properties can be determined and monitored over time, including conditions related to stress and defects. The present invention satisfies that demand.